Merge branch 'msj-retrieve-certify-metadata' into 'master'

Update metadata for Plan:Certify documentation

See merge request gitlab-org/gitlab!47649

doc/administration/reply_by_email.md

 ---
 stage: Plan
-group: Product Planning
+group: Certify
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
 ---
 

doc/user/project/requirements/index.md

 ---
 type: reference, howto
 stage: Plan
-group: Product Planning
+group: Certify
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
 ---
 

doc/user/project/service_desk.md

 ---
 stage: Plan
-group: Product Planning
+group: Certify
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
 ---